The present invention is directed to determining propagation time of a signal and/or line-of-sight (LOS) signal propagation time. More particularly, the invention allows determining the distance between stations and also the location of a station.
Systems for measuring the line-of-sight (LOS) distance between wireless stations have been developed. For example, the Global Positioning System (GPS) represents an already widespread application. However, it represents a self-contained system of providing worldwide geolocation information without the possibility of transmitting user data. Within buildings where most of the envisioned applications are targeted GPS signals may often not be received.
Wireless local area networks (LANs) such as IEEE 802.11x or Hiperlan have been designed for data communication and have found widespread acceptance and proliferation in the industry. However, no provisions have been made for augmenting these systems with geolocation functions. A particular disadvantage is the lack for accurate determination of timing events with respect to propagation time measurements. The IEEE 802.11 MAC specifications relies on a timing resolution for network synchronization and medium access of 1 xcexcs. This corresponds to a propagation distance of 300 m. Furthermore, SIFS (Short InterFrame Space) as the shortest time interval between two frames on the medium is defined as SIFS=16 xcexcs with a tolerance of xc2x10.9 xcexcs according to IEEE 802.11a. This timing resolution, and even more so the corresponding absolute tolerances, render it useless for extracting sufficiently accurate timing information to support geolocation functions.
From the above follows that there is still a need in the art for an improved determination of accurate and sufficient timing information with respect to a signal or frame exchange between stations. Moreover, the determination of the position location of a station within a local area network with an accuracy of a few meters is desired.
Thus the present invention provides methods and apparatus for determining propagation time of a signal (transmit signal) transmittable via a channel, such as a request signal and a response signal. In an example embodiment, an apparatus comprises a timing-control unit for starting a local timer providing a local counter value when transmission of the request signal begins, a correlator for determining a timing reference or time event within the reception of the response signal, and a general-control unit coupled to the timing-control unit and the correlator for stopping the local timer in response to the detection of the timing reference by the correlator. Often, the general-control unit is adapted to derive the propagation time of the signal based on the local counter value, a remote counter value that depends on the request signal, and a determinable time-delay value. The remote counter value represents an inter-time-delay between the reception of the request signal and the start of transmission of the response signal. The propagation distance between a requesting station and a responding station, can be calculated easily by multiplying the propagation time, with the speed of light.
The present invention solves the problem of inaccurate and insufficient timing information with respect to a frame exchange separated by a short interframe space (SIFS) interval. Sufficiently accurate timing events relating to the reception of the frames are derived from correlation peaks generated by the correlation of frame-inherent training sequences, and time delays are implemented with hardware-controlled clocks using length of frame and data rate parameters transmitted during the frame exchange. The invention in its basic form requires no modification of the IEEE 802.11 standard. Position location with accuracy of a few meters can be achieved.
In general, the invention allows the furnishing of an existing and well-established wireless local area network (LAN) technology with an additional geolocation function for establishing the location of participating stations or mobile terminals. In particular, the invention relates to an apparatus and method for measuring the line-of-sight (LOS) signal propagation time, and hence the distance, between two stations, operating for example, under the IEEE 802.11 wireless LAN protocol. With a sufficient number of distance measurements between, for example, a mobile station and other stations or access points (APs) with known position coordinates the position of the mobile station can be determined.
In accordance with a second aspect of the present invention, there is provided a method for determining a propagation time of a signal (transmit signal) transmitted from a first location to a second location as a request signal and received as a response signal by the first location via a channel. In an example embodiment, a method comprises the step of determining the propagation time of the signal based on a local counter value that represents the time between transmission of the request signal and reception of the response signal, a remote counter value that depends on the request signal and being known to the first location, and a determinable time-delay value. The remote counter value represents an inter-time-delay between the reception of the request signal and the start of transmission of the response signal at the second location.
In accordance with a third aspect of the present invention, there is provided a method for determining a propagation time of a signal transmitted from a first location to a second location as a request signal and received as a response signal by the first location via a channel. An example method comprises the steps of starting a local timer providing a local counter value at the first location when transmission of the request signal begins, determining a timing reference within the reception of the response signal, stopping the local timer in response to the detection of the timing reference, and deriving the propagation time of the signal based on the local counter value, a remote counter value depending on the request signal and being known to the first location, and a determinable time-delay value. Thereby, the remote counter value represents an inter-time-delay between the reception of the request signal and the start of transmission of the response signal at the second location.